Method of Distancing a Bubble and Bubble Displacement Apparatus

ABSTRACT

In the field of immersion lithography, it is known to provide a liquid between an optical exposure system and a wafer carrying layers of photosensitive material to be irradiated with a pattern by the optical exposure system. However, bubbles are known to form or exist in the liquid, sometimes close to a surface of the wafer resulting in scattering of light emitted from the optical exposure system. The scattering causes the pattern recorded in the layers of photosensitive material to be corrupted, resulting in defective wafers. Therefore, the present invention provides a bubble displacement apparatus comprising a drive signal generator for driving a force generator arranged to generate a force in response to a drive signal generated by the drive signal generator. The force generated urges the bubble away from the surface.

FIELD OF THE INVENTION

This invention relates to a method of distancing a bubble of the type,for example, that resides close to or in contact with a surface, such asa surface of photosensitive material. The present invention also relatesto a bubble displacement apparatus.

BACKGROUND OF THE INVENTION

In the field of semiconductor processing, photolithography is a widelyemployed technique to “pattern”, i.e. define a profile in one or morelayer of semiconductor material, a semiconductor wafer. Using thistechnique, hundreds of Integrated Circuits (ICs) formed from an evenlarger number of transistors can be formed on a wafer of silicon. Inthis respect, for each wafer, the ICs are formed one at a time and on alayer-by-layer basis.

For about the last four decades, a photolithography apparatus, sometimesknown as a cluster or photolithography tool, has been employed to carryout a photolithographic process. The cluster comprises a track unit thatprepares the wafer, including providing layers of photosensitivematerial on the surface of the wafer prior to exposure to a patternedlight source. To expose the wafer to the patterned light source, thewafer is transferred to an optics unit that is also part of the cluster.The patterned light source is generated by passing a beam of lightthrough a chrome-covered mask, the chrome having been patterned with animage of a given layer of an IC to be formed, for example, transistorcontacts. Thereafter, the wafer is returned to the track unit forsubsequent processing including development of the layers ofphotosensitive material mentioned above.

The wafer, carrying the layers of photosensitive material, is supportedby a movable stage. A projection lens focuses the light passing throughthe mask to form an image on a first field over the layers ofphotosensitive material where an IC is to be formed, exposing the fieldof the layers of photosensitive material to the image and hence“recording” the pattern projected through the mask. The image is thenprojected on another field over the layers of photosensitive materialwhere another IC is to be formed, this field over the layers ofphotosensitive material being exposed to the projected image, and hencepattern.

The above process is repeated for other fields where other ICs are to beformed. Thereafter, the wafer is, as mentioned above, returned to thetrack unit, and the exposed layers of photosensitive material, whichbecome soluble or insoluble through exposure depending upon thephotosensitive materials used, are developed to leave a “photoresist”pattern corresponding to a negative (or positive) of the image of alayer of one or more ICs to be created. After development, the waferundergoes various other processes, for example ion implantation, etchingor deposition. The remaining layers of photosensitive material are thenremoved and fresh layers of photosensitive material are subsequentlyprovided on the surface of the wafer depending upon particularapplication requirements for patterning another layer of the one or moreICs to be formed.

In relation to the patterning process, the resolution of the scannerimpacts upon the width of wires and spaces therebetween that can be“printed”, the resolution being dependent upon the wavelength of thelight used and inversely proportional to a so-called “numericalaperture” of the scanner. Consequently, to be able to define very highlevels of detail a short wavelength of light is required and/or a largenumerical aperture.

The numerical aperture of the scanner is dependent upon the product oftwo parameters. A first parameter is the widest angle through whichlight passing through the lens can be focused on the wafer, and a secondparameter is the refractive index of the medium through which the lightpasses when exposing the layers of photosensitive material on the wafer.

Indeed, to provide the increased resolution demanded by thesemiconductor industry, it is known to reduce wavelengths of light usedwhilst also making lenses bigger to increase the numerical aperture.However, the limits to which the wavelengths of light used can bereduced are rapidly being reached, since wavelengths of less that 157 nmare absorbed by the lenses used.

Additionally, the above-described scanner operates in air, air having arefractive index of 1, resulting in the scanner having a numericalaperture between 0 and 1. Since the numerical aperture needs to be aslarge as possible, and the amount the wavelength of light can be reducedis limited, an improvement to the resolution of the scanner has beenproposed that, other than by increasing the size of the lens, uses thescanner in conjunction with a medium having a refractive index greaterthan that of air, i.e. greater than 1. In this respect, the more recentphotolithographic technique proposed, employing water and known asimmersion lithography, can achieve higher levels of device integrationthan can be achieved by air-based photolithography techniques.

Therefore, scanners employing this improvement (immersion scanners)continue to use low wavelengths of light, but the water provides arefractive index of 1.4 between the lens and the wafer, therebyachieving increased resolution through increasing the numerical apertureof the immersion lithographic apparatus by a factor of 1.4.

Further, the refractive index of the water is very close to that offused silica from which some lenses are formed, resulting in reducedrefraction at the interface between the lens and the water. The reducedrefraction allows the size of the lens to be increased, therebyincreasing the numerical aperture further.

Whilst it appears that immersion lithography can achieve waferthroughputs comparable to air lithography, difficulties exist whenintroducing water between the lens and the wafer. One way of placingwater between the lens and the wafer involves injecting a small film ofwater between the wafer and the lens, the film covering a field over thesurface of the layers of photosensitive material where a given IC is tobe formed, rather than the entire wafer.

However, by placing water between the lens and the wafer, and indeed incontact with an uppermost surface of the layers of photosensitivematerial, defects can be introduced. Such defects, or contaminants, whenin the focal plane of the optics unit (sometimes known as a “scanner”)affect the ability of an immersion lithography apparatus to printdefect-free lines and spaces. In this respect, defect levels can beaffected by particle impurities in the water, temperature variations ofthe water, and thickness uniformity of the water layer. Additionally,bubbles can form in the water layer, which can scatter the light fromthe lens, causing blurring and distortion of the projected image.Possible sources of bubbles are air dissolved in the water, airintroduced into the water as it is ejected from nozzles, air introducedthrough turbulence caused by movement of a stage or projection system,and/or “out gassing”, the egress of gas from the photosensitive materialas a result of impurities or simply the components of the photosensitivematerial. Whilst so-called “degasification” can remove bubbles caused byair dissolved into the water and careful nozzle design can eliminate thenozzle-induced bubbles, air trapped on the uppermost surface of thelayers of photosensitive material can also create bubbles as the waterflows over the surface of the wafer.

One proposed solution to avoid imaging problems caused by bubblesadjacent the surface of the wafer is to apply a top anti-reflectioncoating over the layers of photosensitive material prior to placing thewafer in the immersion lithography apparatus, thereby creating a barrierbetween bubbles and the uppermost surface of the layers of thephotosensitive material. However, such a solution, whilst keepingbubbles further from the focal plane, i.e. the uppermost surface of thelayers of photosensitive material, serves to increase the focal distanceand hence reduce the resolution that can be achieved.

STATEMENT OF INVENTION

According to the present invention, there is provided a bubbledisplacement apparatus and a method of distancing a bubble as set forthin the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

At least one embodiment of the invention will now be described, by wayof example only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a bubble displacement apparatusconstituting a first embodiment of the invention;

FIG. 2 is a schematic diagram of another bubble displacement apparatusconstituting a second embodiment of the invention; and

FIG. 3 is a schematic diagram of further bubble displacement apparatusconstituting a third embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Throughout the following description identical reference numerals willbe used to identify like parts.

Referring to FIG. 1, a semiconductor wafer 100 having layers ofphotosensitive material disposed thereon, the layers of photosensitivematerial having an upper surface 102, is disposed upon a substrate stage(not shown) of an immersion lithography apparatus. In this example, theimmersion lithography apparatus is a modified TWINSCANT™ XT:1250ilithography scanner available from ASML. The lithography scanner is acomplex apparatus having many parts, the structure and operation ofwhich, are not directly relevant to the embodiments disclosed herein.Consequently, for the sake of clarity and conciseness of description,only the parts of the lithography scanner of particular relevance to theembodiments herein will be described.

The immersion lithography apparatus comprises an optical exposure(projection or catadioptric) system 104 connected to a liquid supplysystem 106, sometimes known as a “showerhead”. Liquid 107 is disposedbetween the bottom of the optical exposure system 104 and the surface102 of the layers of photosensitive material.

The liquid supply system 106 comprises water inlet/outlet ports 108 influid communication with a reservoir 109 defined by an inner peripheralsurface 110 of the liquid supply system 106 and the upper surface 102. Avacuum pump (not shown) is coupled to vacuum ports 112, the vacuum ports112 being in fluid communication with a first channel loop 114. Acompressor (not shown) is coupled to air supply ports 116, the airsupply ports 116 being in fluid communication with a second channel loop118.

A first electrode 120, for example a copper electrode, is coupled to theliquid supply system 106 so that the first electrode 120 is in contactwith the liquid 107, and a second electrode 121, for example also acopper electrode, is coupled to a peripheral edge of the surface 102 ofthe wafer 100, and hence the photosensitive materials. The first andsecond electrodes 120, 121 are coupled to a driver circuit 122. In thisexample, although not shown in FIG. 1, the second electrode comprises anumber of circumferentially spaced electrical contacts to provide auniform charge across the surface 102 of the wafer 100.

In operation, at least one bubble 124 is disposed within the liquid 107and in close proximity to the surface 102 of the layers ofphotosensitive material. The optical exposure system 104 coupled to theliquid supply system 106 scans the surface 102 of the layers ofphotosensitive material in order to project a pattern onto the layers ofphotosensitive material in a manner known for the lithography scanner. Avacuum provided to the first channel loop 114 via the vacuum ports 112and pressurised air expelled into the second channel loop 118 via theair supply ports 116 prevent egress of the liquid 107 from the reservoir109 through a clearance 126 between the liquid supply system 106 and thesurface 102, thereby serving as a seal and a means of preventingspurious deposition of the liquid 107 on the surface 102.

The proximity of the bubble 124 to the surface 102 will result inscattering of light emitted from the optical exposure system 104.However, a charge, in this example negative, inherently surrounds thebubble 124 and so the driver circuit 122 generates a substantiallycontinuous positive driver signal that is applied to the electrode 120.The positive charge at the electrode 120 serves to electrostaticallyattract the negatively charged bubble 124 to the housing of the liquidsupply system 106, thereby causing the bubble 124 to move away from thesurface 102 and hence the bubble 124 is distanced from the surface 102.

In this example, the driver circuit 122 generates the continuouspositive voltage signal. However, it should be appreciated that othersignals can be generated by the driver circuit 122, for example acontinuous negative signal. Alternatively, the driver signal can be atime-varying signal, for example a modulated signal or a pulsed signal.In all cases, an electrostatic force is used to urge the bubble 124 awayfrom the surface 102. Of course, the skilled person will appreciate thatthe first electrode 120 can be disposed within the liquid 107 atpositions, which urge the bubble 124 laterally out of the field.

In another embodiment (FIG. 2), another type of force is used to urgethe bubble 124 away from the surface 102 of the layers of photosensitivematerial. A first electromagnet 200 and a second electromagnet 202 aretherefore coupled to the driver circuit. However, in order to enable thefirst and second electromagnets 200, 202 to mechanically influence thebubble 124, the bubble 124 is subjected to an initial additional force,for example by an electrostatic field created by the first and secondelectrodes 120, 121 described above, although alternative mechanisms canbe employed to create the initial movement of the bubble 124. The firstand second electromagnets 200, 202 are arranged either side of theimmersion lithography apparatus with opposite poles facing each other sothat, when energised, a magnetic field can be generated across thereservoir 109.

In operation, the driver circuit 122 generates a driver signal to causethe first and second electromagnets to generate a substantiallycontinuous, and optionally uniform magnetic field therebetween andacross the reservoir 109.

When the first and second electromagnets 200, 202 are energised, thenegatively charged bubble 124 is urged, by the magnetic field generated,away from the surface 102 of the layers of photosensitive material.

In this example, the driver signal is generated so that a continuousfield is generated between the first and second electromagnets 200, 202in one direction. However, it should be appreciated that other signalscan be generated by the driver circuit 122 so that, for example, thecontinuous field can be generated in an opposite direction to thatdescribed above. Alternatively, the driver signal can be a time-varyingsignal, for example a modulated signal or a pulsed signal, resulting ina corresponding magnetic field being generated between the first andsecond electromagnets 200, 202. In all cases, a magnetic force is usedto continue moving the bubble 124 away from the field at the surface102. Of course, the skilled person will again appreciate that the firstand second electromagnets 200, 202 can be disposed about the liquidsupply system 106 so as to urge the bubble 124 laterally out of thefield.

In a further embodiment (FIG. 3), yet another type of force is used tourge the bubble 124 away from the surface 102 of the layers ofphotosensitive material. In this respect, instead of coupling theelectrode 120 or the first and second electromagnets 200, 202 to thedriver circuit 122, a piezoelectric transducer 300 is coupled to anunderside 302 of the wafer 100, the piezoelectric transducer 300 beingelectrically coupled to the driver circuit 122.

In operation, the driver circuit 122 generates a time-varying signal,the time-varying signal being, in this example, a modulated signal,although the skilled person will appreciate that other time-varyingsignals can be employed, such as a pulse signal. The time-varying signalis of a predetermined amplitude and/or frequency, resulting in thepiezoelectric transducer 300 generating acoustic waves in the liquid 107corresponding to the form of the time-varying signal. In this example,the amplitude and/or frequency of the time-signal is sufficient to urgethe bubble 124 away from the surface 102 of the layers of photosensitivematerial by the application of a mechanical force on the bubble 124 bythe acoustic waves in the liquid 107.

In yet a further embodiment, the amplitude and/or frequency of thetime-varying signal is sufficiently great to cause the bubble 124 tobreak-up into a number of smaller bubbles (not shown), the number ofbubbles created being sufficiently small such that the proximity of thenumber of bubbles created to the surface 102 does not result inscattering of light emitted from the optical exposure system 104. Inthis respect, the skilled person will appreciate that the intensityand/or profile of the waveform used to disperse the bubble 124 dependsupon a number of parameters, for example size of the bubble 124, volumeof the medium in which the bubble 124 is disposed (in this example, theliquid 107), flow rate of the medium and/or concentration of bubbles,where more than one bubble is being dispersed.

Although the above two embodiments employ a piezoelectric transducer, itshould be appreciated that any device capable of generating acousticwaves in the liquid 107 can be employed.

Throughout the description, reference has been made herein to the bubble124. However, the skilled person will understand that more than onebubble can exist in the liquid 107. In the present description, a singlebubble has only been described for the purpose of preserving simplicityof description.

Although not explicitly described above, the skilled person willappreciate that any combination of the above described ways ofgenerating forces for moving the bubble 124 can be employed.

Although reference to “scanners” (Step and Scan Systems) is made herein,the skilled person will appreciate that alternative optical exposuresystems can be employed, for example a so-called “stepper” (step andrepeat) in which a reticle passes between a light source and a lenssystem.

It is thus possible to provide a bubble displacement apparatus and amethod of displacing bubbles that permits immersion lithography to be aviable lithographic technique, reducing occurrences of defects andproviding a wider range of photolithographic process parameters thanavailable to existing photolithographic tools. Consequently, higheryields of wafers can be produced.

1. A bubble displacement apparatus for distancing a bubble in a fluidfrom a surface in an immersion optical exposure system, the apparatuscomprising: a drive signal generator arranged to generate, when in use,a drive signal; and a force generator coupled to the drive signalgenerator; wherein the force generator is arranged, when in use, toapply at least one force for urging the bubble away from the surface,the force generator being arranged to generate at least one field, andthe at least one field is a magnetic field.
 2. An apparatus as claimedin claim 1, wherein the at least one field in an electric field.
 3. Anapparatus as claimed in claim 1, wherein the force generator is arrangedto generate at least an acoustic wave.
 4. An apparatus as claimed inclaim 1, wherein the drive signal is a time-varying signal.
 5. Anapparatus as claimed in claim 4, wherein the time varying signal is amodulated signal.
 6. An apparatus as claimed in claim 4, wherein thetime-varying signal is a pulsed signal.
 7. A method of distancing abubble in a fluid from a surface for irradiation by an immersion opticalexposure system, the method comprising the steps of: generating a drivesignal; generating a magnetic field so as to provide a force in responseto the drive signal, thereby urging the bubble away from the surface. 8.An apparatus as claimed in claim 2, wherein the drive signal is atime-varying signal.
 9. An apparatus as claimed in claim 3, wherein thedrive signal is a time-varying signal.
 10. A method as claimed in claim7, further comprising generating an acoustic wave.
 11. A method asclaimed in claim 7, wherein the drive signal is a time-varying signal.12. A method as claimed in claim 11, wherein the time varying signal isa modulated signal.
 13. A method as claimed in claim 11, wherein thetime-varying signal is a pulsed signal.